I was just reading a question on cord and plug connected air compressors at 'doityourself.com', when I remembered this thread. So I went and did some code re-reading and some calculating.
1) Air compressors, like vacuum cleaners and stereo amplifiers, often have their ratings 'pumped up' by the marketing department. I've seen '5.0 Hp' vacuum cleaners with 5-15 cord caps on their power cords.
2) 22.4A * 240V = 7.2 Hp if you presume 100% power factor and 100% efficiency, neither of which is true. Therefore this air compressor is _not_ a 7.5 Hp machine.
3) Take a look at 430.6(A)(1) exception #3: "For a listed motor-operated appliance that is marked with both motor horsepower and full-load current, the motor full-load current marked on the name-plate of the appliance shall be used instead of the horsepower rating on the appliance nameplate to determine the ampacity or rating of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short circuit and ground-fault protection, and any separate overload protection."
I presume that the difference is that a listed appliance has a much better defined mechanical load placed on the motor, so the listing agency can limit the startup overload and less strain is expected on the supply components.
If this compressor were sold as an integral unit with the motor built in, and the name-plate current is 22.4A, then I would think it would be just fine on a 6-30 or L6-30 receptacle.
Could someone else confirm this interpretation?
-Jon
